This invention is directed to an absorbent binder or coating composition, and a method of making the absorbent binder or coating composition.
Adhesives, or binders, are a necessary element of many absorbent products. While adhesives beneficially hold products together, adhesives may also have a tendency to interfere with the absorbency of fluids in absorbent products. Adhesives are typically hydrophobic and therefore are not conducive to absorbency or liquid transfer functions. Furthermore, most adhesives are non-absorbent and thus serve no liquid retention function.
Hydrophilic adhesives are known, such as adhesives formulated from water-soluble polymers such as poly(vinyl alcohol), poly(vinyl methyl ether), poly(vinyl pyrrolidone), poly(ethylene oxide), or cellulose derivatives such as hydroxypropyl cellulose. Dextrans, starches and vegetable gums have been used to provide hydrophilic adhesives. These materials provide adhesion under dry conditions. However, upon exposure to aqueous fluids, these materials lose bonding capability because they are substantially soluble in aqueous fluids.
A known approach for making hydrophilic adhesives more functional upon exposure to aqueous fluid is to crosslink the water-soluble polymers. As a result of crosslinking, the material becomes swellable, and no longer soluble, in aqueous fluid. However, crosslinked polymers are difficult to apply to substrates or to establish intimate contact with surfaces because the crosslinked polymers are solid materials and have little or no ability to flow.
What is therefore needed is a hydrophilic binder or coating that has latent crosslinking capability. Such binder or coating could be easily applied, like a water-soluble polymer, since the hydrophilic binder or coating would be capable of flow prior to crosslinking. Latent crosslinking capability would also provide a simple means of crosslinking the polymer after the polymer has established intimate contact with substrates or has formed a desired final shape or form.
Post-application crosslinking is well known. Typical means of inducing the formation of crosslinks include high temperature xe2x80x9ccuringxe2x80x9d or exposure to radiation, such as ultraviolet or gamma radiation. Another known means of post-application crosslinking is moisture-induced crosslinking.
Recent development efforts have provided coating materials for a variety of uses. For example, U.S. Pat. No. 6,054,523, to Braun et al., describes materials that are formed from organopolysiloxanes containing groups that are capable of condensation, a condensation catalyst, an organopolysiloxane resin, a compound containing a basic nitrogen, and polyvinyl alcohol. The materials are reported to be suitable for use as hydrophobic coatings and for paints and sealing compositions.
Anderson et al., in U.S. Pat. No. 5,196,470, reported an alcohol-based, water-soluble binder composition. Because this composition is water-soluble and not cross-linked, it has no absorbency.
Others have reported the production of graft copolymers having silane functional groups that permitted the initiation of cross-linking by exposure to moisture. Prejean (U.S. Pat. No. 5,389,728) describes a melt-processible, moisture-curable graft copolymer that was the reaction product of ethylene, a 1-8 carbon alkyl acrylate or methacrylate, a glycidyl containing monomer such as glycidyl acrylate or methacrylate, onto which has been grafted N-tert-butylaminopropyl trimethoxysilane. The resulting copolymers were reported to be useful as adhesives and for wire and cable coatings.
Furrer et al., in U.S. Pat. No. 5,112,919, reported a moisture-crosslinkable polymer that was produced by blending a thermoplastic base polymer, such as polyethylene, or a copolymer of ethylene, with 1-butene, 1-hexene, 1-octene, or the like; a solid carrier polymer, such as ethylene vinylacetate copolymer (EVA), containing a silane, such as vinyltrimethoxysilane, and a free-radical generator, such as an organic peroxide; and heating the mixture. The copolymers could then be cross-linked by reaction in the presence of water and a catalyst, such as dibutyltin dilaurate, or stannous octoate.
U.S. Pat. No. 4,593,071 to Keough reported moisture cross-linkable ethylene copolymers having pendant silane acryloxy groups. The resultant cross-linked polymers were reported to be especially resistant to moisture and to be useful for extruded coatings around wires and cables. The same group has reported similar moisture curable polymers involving silanes in U.S. Pat. Nos. 5,047,476, 4,767,820, 4,753,993, 4,579,913, 4,575,535, 4,551,504, 4,526,930, 4,493,924, 4,489,029, 4,446,279, 4,440,907, 4,434,272, 4,408,011, 4,369,289, 4,353,997, 4,343,917, 4,328,323, and 4,291,136.
U.S. Pat. No. 5,204,404 to Werner reported crosslinkable hydrophobic acrylate ester copolymers including 0.1 to 10% acrylic acid. The resultant cross-linked polymers were reported to be useful for painting and refinishing the exterior of automobiles.
These examples of moisture-induced crosslinking are applied to substantially hydrophobic polymers. Since the cured products of these formulations are reported to be useful for coverings for wire and cable, and for non-conductive coatings for electrical conductors, and for painting and refinishing the exterior of automobiles, it would be expected that they are durable coatings for which properties such as water absorbency would be a disadvantage.
There is thus a need within the field of absorbent products for absorbent binders, adhesives, or coatings. Furthermore, there is a need within the field of absorbent products for such absorbent binders, adhesives, or coatings that can be prepared by post-application, moisture-induced crosslinking of hydrophilic polymers.
In response to the discussed difficulties and problems encountered in the prior art, a new absorbent composition, useful as a binder, adhesive, or coating material, has been discovered. The absorbent composition includes a hydrophilic polymer which has the capability of post-application, moisture-induced crosslinking. This capability provides for absorbent products having greater absorbent capacity.
The absorbent binder composition includes at least 15 mass percent monoethylenically unsaturated polymer, such as carboxylic acid, sulphonic acid, phosphoric acid, or salts thereof, and an acrylate or methacrylate ester that contains an alkoxysilane functionality. Upon exposure to water, the alkoxysilane functionality forms a silanol functional group which condenses to form a crosslinked polymer. Thus, the absorbent binder composition provides enhanced adhesion in a wet condition, as well as absorbency.
The absorbent binder composition suitably has a glass transition temperature below about 30 degrees Celsius, or below about 10 degrees Celsius, and a bending modulus lower than the bending modulus of a substrate to which the composition is applied. The absorbent binder composition may be used in the manufacture of absorbent products, and therefore may be applied to such substrates as nonwoven webs, woven webs, knitted fabrics, cellulose tissue, plastic film, stranded composites, elastomer net composites, or any other suitable substrates. Examples of suitable types of plastic film substrates include those made of polypropylene, low density polyethylene, high density polyethylene, linear low density polyethylene, and ultra low density polyethylene. Examples of absorbent articles in which the absorbent binder composition may be used include diapers, diaper pants, training pants, feminine hygiene products, incontinence products, swimwear garments, and the like.
The absorbent binder composition of the invention can be made by polymerizing monoethylenically unsaturated monomers, one of which contains an alkoxysilane functionality. The polymerization may be induced by a variety of initiation techniques including thermal initiation, radiation initiation, or redox chemical reactions. Various types of effective radiation initiation include ultraviolet, microwave, and electron-beam radiation. The initiator generates free radicals to cause polymerization of the monomers. The resultant copolymer includes latent moisture-induced crosslinking capability by incorporation of the alkoxysilane functionality. This copolymer may be applied, in a flowable state, to a substrate or other end use application. Moisture-induced crosslinking may be accomplished through hydrolysis of the alkoxysilane and subsequent condensation upon removal of the solvent from the substrate, either by evaporation of the solvent from the substrate or using any other effective technique. Alternatively, the hydrolysis of the alkoxysilane and subsequent condensation may occur after solvent removal by exposure of the coating to moisture in ambient air.
With the foregoing in mind, it is a feature and advantage of the invention to provide an absorbent binder composition, and a method of making such absorbent binder compositions.